Generally, in an electron beam apparatus which is configured to irradiate a sample applied with a retarding voltage with a primary electron beam and detect secondary electrons emitted from an irradiated spot to produce an image of the sample, a larger electric field must be applied between the surface of the sample and an objective lens in order to reduce axial chromatic aberration. However, as the electric field strength is increased on the surface of the sample with the intention to reduce the axial chromatic aberration, a discharge will occur between the sample and objective lens. For this reason, the electric field strength must be set to be relatively small, resulting in a problem of relatively large axial chromatic aberration.
Also, an electron beam apparatus is known (see, for example, JP-A-2004-214044) for shaping an electron beam emitted from an electron gun into a rectangular shape, irradiating a sample with the shaped electron beam, producing an enlarged image from secondary electrons emitted from the sample using a projection optical lens, and detecting the sample image using a detector such as a TD1 detector. This apparatus employs an electrostatic lens as an objective lens for irradiating a sample with an electron beam (see, for example, JP-A-11-132975, Republished WO2002/045153). Also, a secondary electron projection optical system is known to employ an electrostatic lens having five electrodes, and further in a lithography apparatus, an electro-optical system which satisfies a MOL (Moving Object Lens) condition is also known.
When an electrostatic lens is employed for an objective lens, a higher voltage applied to a central electrode of the electrostatic lens results in a higher susceptibility to a discharge. Accordingly, when a low voltage is employed, axial chromatic aberration increases. When the electric field strength is increased in order to reduce the axial chromatic aberration, a problem arises in that a discharge occurs between a sample and the lens. Anyway, the transmittance of secondary electrons is difficult to be increased, as a consequence.
The employment of a magnetic lens for an objective lens is also known (for example, see JP-A-2003-168385 and JP-A-2003-173756). In this event, problems lie in that an axial magnetic field is not zero on the surface of a sample, and secondary electron beams emitted from the sample in the normal direction do not intersect with the optical axis or pass through an NA aperture. Also, a MOL scheme has problems that a beam must be scanned, and it is hardly compatible with an inspecting device. Further, in a rectangular visual field, problems are a long distance from the optical axis for the area, and large astigmatism. Another problem lies in that a second electron beam is blurred by space charges of a primary electron beam.
From the foregoing, a need exists for an electron beam apparatus which is free from a discharge occurring between a sample and an electrostatic lens, can reduce axial chromatic aberration and various aberration, can force secondary electrons to intersect with an optical axis and to pass through an NA aperture, can vary the magnification of an image by secondary electrons, can improve the transmittance of the secondary electrons, and can reduce the occurrence of blurs of the secondary electrons due to a space charge effect of primary electrons, and for a semiconductor device manufacturing method using the apparatus.
Conventionally, when an LaB6 cathode is operated in a space charge limiting region, there is an advantage of small shot noise, but on the contrary, there is a problem of large chromatic aberration due to a large energy width. A technology for correcting an objective lens for axial chromatic aberration using a plurality of quadrupole lenses in order to accomplish a high resolution of several nm to one nm has been practically used in SEM and transmission electron microscope.
On the other hand, with the trend of higher integration of semiconductor devices and increasing miniaturization of patterns, inspection apparatuses have been required to provide higher resolution and higher throughput. In order to examine a wafer substrate of 100-nm design rule for defects, it is necessary to view the presence/absence of pattern defects and particles in wires having a line width of 100 nm or less, defective vias, and electric defects thereof. Accordingly, a resolution of 100 nm is required, and a higher throughput is also required because the amount of inspects is increased due to an increase in manufacturing steps resulting from higher integration of devices.
As devices are formed of a larger number of layers, an inspection apparatus is also required to provide a function of detecting defective contacts (electric defects) of vias which connect wires between layers. It is anticipated that an electron beam based defect inspection apparatus will go mainstream in place of an optical defect inspection apparatus in regard to the resolution and defective contact inspection. However, the electron beam based defect inspection apparatus is disadvantageously inferior to the optical one in regard to the throughput. As such, a need exists for the development of an electron beam based inspection apparatus which is capable of a high resolution, a high throughput, and detecting electric defects.
It is said that the resolution of the optical inspection apparatus is limited to one half of the wavelength of used light, and the resolution is approximately 0.2 μm in a practiced example. On the other hand, in a scheme based on electron beams, a scanning electron beam scheme (SEM scheme) has been brought into practical use, where the resolution is 0.1 μm and an inspecting time is eight hours per wafer (200 mm wafer). In addition, the electron beam scheme is largely characterized by its abilities to inspect for electric defects (disconnected wires, defective conduction, defectively connected vias, and the like), but merely provides a very low inspecting speed. Therefore, the development of a defect inspection apparatus which provides a high inspecting speed is expected.
Further, a known electron beam apparatus irradiates a sample with an electron beam having a rectangular cross-section, enlarges secondary electrons emitted from the sample, focuses the enlarged secondary electrons onto a detection plane, and inspects the surface of the sample (see, for example, JP-2002-216694). However, since this type of electron beam apparatus has large axial chromatic aberration, the throughput must be largely reduced in order to provide an S/N ratio required to evaluate at a high resolution.
Another known electron beam apparatus scans the surface of a sample with a plurality of beams, and detects secondary electrons from the sample using a plurality of detectors to increase the throughput (see, for example, U.S. Pat. No. 5,892,224). However, in scanning a plurality of beams, no clear solution has been provided in regard to how a plurality of beams should be arranged in order to most effectively perform evaluations. Moreover, the electron beam apparatus has a problem in that if a magnetic lens is employed for an objective lens, secondary electrons emitted from a sample in the normal direction to the surface of the sample do not intersect with the optical axis.
Also, while it is known to produce an image at an ultra-high resolution of 1 nm or less by correcting axial chromatic aberration, an increase in beam strength has not been practiced, instead of improving the resolution by the correction of aberration.